When Kate of Sketchbook mentioned being unable to distinguish ri, so, and n, I thought she was playing to stereotype. Today, however, Steven admitted to mixing them in Ken's comments. But it's simple, in the same way distinguishing Sumire and Ami was simple.

The process takes up to two steps. First, determine if this is "ri".

If the short stroke is vertical, or parallel to (part of) the long stroke, it is "ri".

Second, in case the short stroke is crooked, is to split "so" from "n".

If the short stroke is aligned with the top of the long stroke, it's "so". If it is aligned with the left end, it's "n".

No, I don't know what "stone" is about. Probably the name of the store.